The present invention is directed towards a method and apparatus for improving the efficiency of the modulation stage of a transmitter.
The dependence of the anode current I.sub.a of the modulator tube upon the grid voltage U.sub.g is represented, at least in a certain range, by the expression:
I.sub.a =k.multidot.U.sub.g.sup.m, where m.apprxeq.1.5 and k is a material constant.
The modulation systems generally used today, anode modulation with push-pull Class AB modulation amplifiers, provide very stable operation, without problems, from the point of view of modulation characteristics. In order to keep the coefficient of harmonic distortion within reasonable limits, the grid bias of the modulator tubes must be adjusted so that the resulting control characteristic curve is linear. This leads to relatively high quiescent currents (see FIG. 1) and hence to the corresponding pure power dissipation: EQU P.sub.VO =(I.sub.O '+I.sub.O ").multidot.U.sub.a
With the present-day trend towards energy saving measures, however, other circuits are now being sought, for example pulse modulators, in order to improve the total efficiency of the transmitter. This approach is not without problems and the expense is high.
Since the above-mentioned power dissipation P.sub.VO results mainly from the measures to reduce the coefficient of harmonic distortion, this invention proposes a circuit in which this unwanted quiescent power can be considerably reduced by means of an appropriate deemphasis of the input signal and dispensing with any linearization of the characteristics of the output tubes.